1. Field of the Invention
The present invention relates to a backlight unit, and more particularly to a backlight unit for reducing light loss caused by an intermediate light guide panel and for reducing sag of the intermediate light guide panel. Further, the present invention relates to a liquid crystal display device for increasing display quality by a use of the backlight unit.
2. Description of the Related Art
Generally, a liquid crystal display device has its application scope on a broadening trend due to characteristics of lightness, thinness, low driving power consumption, etc. Accordingly, the liquid crystal display device is used in office automation equipments, audio/video equipments, etc. The liquid crystal display device controls the transmitted amount of light beam in accordance with a signal applied to a plurality of control switches that are arranged in a matrix pattern, thereby displaying a desired picture on a screen.
The liquid crystal display device is not a self-luminous display device, and requires a separate light source such as a backlight.
The backlight is classified into a direct type and an edge type in accordance with the location of the light source. The edge type backlight has the light source installed at the edge of one side of a liquid crystal display device, and irradiates an incident light from the light source to a liquid crystal display panel through a light guide panel and a plurality of optical sheets. The direct type backlight has a plurality of light sources disposed below the liquid crystal display panel, and irradiates the incident light from the light sources to the liquid crystal display panel through a diffusion plate and a plurality of optical sheets.
The light source used in the backlight is classified into a cold cathode fluorescent lamp CCFL and a light emitting diode (hereinafter, referred to as ‘LED’). Recently, the direct type backlight with high color purity is increasingly used on LCD TVs.
Referring to FIG. 1, a liquid crystal display device of the related art includes a liquid crystal display panel 11 for displaying a picture; and a backlight unit 10 for irradiating light to the liquid crystal display panel 11.
The liquid crystal display panel 11 includes a plurality of data lines, a plurality of scan lines crossing the data lines and liquid crystal cells between upper and lower substrates arranged in an active matrix pattern. Further, in the liquid crystal display panel, pixel electrodes and a common electrode are formed for applying electric field to each of the liquid crystal cells. Thin film transistors (hereinafter, referred to as ‘TFT’) for switching a data voltage applied to the pixel electrode in response to a scan signal are formed in the crossing parts of the data lines and the scan lines. In the liquid crystal display panel, gate drive integrated circuits (hereinafter, referred to as ‘IC’) are electrically connected to gate drive IC's through a tape carrier package (hereinafter, referred to as ‘TCP’).
The backlight unit 10 includes a bottom cover 13; a lower reflection plate 14; an intermediate light guide panel 15; a diffusion plate 16; a plurality of light emitting diodes 17 and a plurality of optical sheets 12.
The bottom cover 13 includes a bottom surface and a side surface, and receives the lower reflection plate 14 thereinside. Holes respectively penetrating the light emitting diodes 17 are formed in the lower reflection plate 14.
The light emitting diodes 17 includes a red light emitting diode, a green light emitting diode and a blue light emitting diode, and generates red, green and blue lights by emitting light by the current supplied from a light source drive circuit (not shown).
The intermediate light guide panel 15 is made from a transparent plastic material, and is disposed between the light emitting diodes 17 and the diffusion plate 16. An intermediate reflection plate 18 corresponding to the light emitting diode in one-to-one relationship is stuck to a bottom surface location corresponding to a location right above the light emitting diode 17 on the intermediate light guide panel 15. The intermediate reflection plate 18 is called as a diverter, and reflects the red, green and blue lights which are directly irradiated from the light emitting diodes 17.
The red, green and blue lights irradiated from the light emitting diodes 17 are mixed by the lower reflection plate 14, the intermediate reflection plate 18 and the intermediate light guide panel 15, and as a result, a white light is incident to the optical sheets 12 through the diffusion plate 16.
The diffusion plate 16 is assembled together with the bottom cover 13 with the intermediate light guide panel 15. The diffusion plate 16 includes a plurality of beads and disperses the light incident through the intermediate light guide panel 15 by use of the beads to prevent brightness difference generated between locations of the light emitting diodes and in other locations in the display surface of the liquid crystal display panel 11.
The optical sheets 12 include a diffusion sheet and a prism sheet, and act to increase brightness in the front of the display surface by uniformly irradiating the light incident from the diffusion plate 16 to the whole liquid crystal display panel 11 and bending the progress path of the light in a vertical direction to the display surface.
The backlight unit 10 of the related art has at least a following problem. The intermediate light guide panel 15 has supporting points only on, the edge of the liquid crystal display device However, as the size of the display device increases, a sag of the intermediate light guide panel 15 becomes more pronounced. When the sag occurs, the gaps between the intermediate reflection plates 18 and the light emitting diodes 17 are varied, and as a result, the amount of the light incident to the liquid crystal display panel 11 is reduced which lowers the brightness. Also the red, green and blue lights generated from the light emitting diodes 17 are not uniformly mixed which decreases the color purity of the white light irradiated from the liquid crystal display panel 11. As a result, a color reproduction characteristic of the liquid crystal display panel 11 deteriorates.